Only for multi CPU mobos (and that would be pinning a thread to a CPU/core with NUMA enabled where a task accessed local ram instead of all system ram). Even then, I think all ram would run at the lowest frequency.
I've never mixed CPUs and RAM speeds. I've only ever worked on systems with matching CPUs and ram modules.

I think the hardware cost and software complexity to achieve this is beyond the cost of "more ram" or "faster storage (for faster swap)"

As to whether it's possible to get certain apps use specific physical RAM sticks, I am not sure, but that seems unlikely and would probably require some very low level modifications to your operating system. But even before you get to that point you'd have to physically connect them to your new motherboard, which will only work if there are both free RAM slots on it, and your new motherboard has slots for the same generation of RAM that your old PC uses.

sort of;; https://www.amazon.com/pci-ram-expansion-card/s?k=pci+ram+expansion+card

Doubt it